1. Field of the Invention
The present invention relates to a signal processing apparatus, and more particularly to an apparatus for generating clocks phase-synchronized with an input signal.
2. Related Background Art
A conventional liquid crystal display apparatus is required to reproduce and display image signals of various kinds with high fidelity, the image signals including those sent from external computers of a different maker and those having different pixel frequencies. In order to satisfy this requirement, an image mode discrimination unit discriminates between image modes based upon the horizontal sync signal HD and vertical sync signal VD of an input image signal to determine a pixel frequency suitable for the image mode and set the frequency to a PLL circuit. By using the horizontal sync signal HD as a reference, the PLL circuit generates clocks having a frequency N times as high as that of the horizontal sync signal HD, and samples the image or video signal for A/D conversion which uses the clocks.
The PLL circuit therefore determines the phase of the sampling clock for A/D conversion by using the horizontal sync signal HD as a reference. However, the phase relation between the horizontal sync signal HD and the image signal is not always constant because distortion, jitter and the like of the horizontal sync signal HD are influenced by differences in image output apparatus such as computers to be connected and differences in connection wiring lengths. It is therefore necessary to set the sampling phase independently for each apparatus to be connected. A shift in the sampling phase is a main factor in image quality degradation, particularly the image quality of a multi-scan display is required to process various image modes. If pixel data is not sampled at its optimum point, the dynamic range and gradation become narrow so that the image quality is conspicuously degraded having low contrast, large noises and the like and a high image quality is difficult to display.
If a horizontal sync signal having an integrated waveform portion is input to a liquid crystal display apparatus, the timing of a reference signal or horizontal sync signal cannot be detected correctly. In this case, if the sampling point is near the border between the preceding and succeeding pixels, uniform pixel data cannot be obtained and flickering may occur because of jitter.
If a plurality of personal computers are connected in a switching manner to a large screen display apparatus of a conference system or education system, it is more difficult to obtain an optimum sampling phase, because the phases of the horizontal sync signal and image signal of each personal computer are different and in addition because the horizontal sync signal becomes unsharp since each personal computer is connected via a long wire to an image signal switching circuit.
It is an object of the present invention to solve the above-described problems.
It is another object of the present invention to control the sampling phase of an image signal to have an optimum phase.
Under such objects, according to an aspect of the present invention, there is provided an image signal processing apparatus comprising: clock generating means for generating a clock phase-synchronized with an input image signal; sampling means for sampling the input image signal in response to the clock; comparison means for comparing a plurality of samples output from the sampling means with each other; and control means for controlling a phase of the clock by controlling the clock generating means in accordance with a comparison result by the comparison means.
The other objects and features of the present invention will become apparent from the following detailed description of the invention when read in conjunction with the accompanying drawings to follow.